1. Field of the Invention
The present invention relates to a high strength cold rolled steel sheet and a high strength zinc-coated steel sheet suitable for use in parts such as automotive panels which require a good external appearance, good workability, and good shape accuracy, i.e., shape retention. The present invention also relates to a steel for preparing such a steel sheet and to a method for manufacturing the steel sheet.
2. Description of the Related Art
Automotive panels and other exterior members of automobiles are required to have an excellent appearance and a good strength exemplified by dent resistance. A primary cause of flaws in the external appearance of such panels is surface strains caused by elastic restoration after press forming. Therefore, a material having a low yield strength is suitable for such panels. However, if the yield strength of a panel after forming is too low, the panel has poor dent resistance, and indentations remain when the panel is pressed with a finger.
Japanese Published Unexamined Patent Application Hei 2-111841(1990) discloses a steel sheet which is soft at the time of forming and which has a yield stress which increases at the time of bake finishing after forming. However, due to a deterioration of strain aging properties of the steel sheet, there is a practical limit to the extent to which the yield stress of that steel sheet can be increased.
A multi-phase structure steel sheet is known to have good strain aging properties and a good bake hardenability. Japanese Published Unexamined Patent Application Hei 4-173945(1992) describes a method for the manufacture of such a steel sheet. However, in order to manufacture a steel sheet with a multi-phase structure, it is necessary to add large amounts of C or Mn, so the yield strength of the steel sheet becomes too high, and it is difficult to use the steel sheet in automotive panels.
Japanese Published Unexamined Patent Application No. 2000-109965 discloses a method of manufacturing a steel sheet having a multi-phase structure and a low yield strength. However, the steel sheet has a low r-value, so it is not completely satisfactory with respect to formability.
The present invention provides a steel suitable for forming cold rolled steel sheet and zinc-coated steel sheet having the ability to undergo aging at room temperature (strain aging), good shape accuracy, good dent resistance, and good press-formability and which can be utilized for exterior members of automobiles. The present invention also provides a method for the manufacture of this steel sheet.
A method of improving the formability of a steel sheet with a multi-phase structure by retaining austenite has already been disclosed in Japanese Published Unexamined Patent Application Hei 11-131145(1999), for example. However, according to that disclosure, in order to obtain retained austenite, it is necessary to add large amounts of Si or Al. In a method in which the amount of bainite is made extremely large, the yield strength becomes too high, and it becomes easy for stretcher strains to occur, so the resulting sheet is not appropriate for application to automotive panels. Furthermore, if the amount of Si is made too high, in hot dip galvanizing, there are problems with respect to the wettability at the time of manufacture and with respect to the ability to perform galvannealing (alloying treatment).
The present inventors found that by adding a suitable amount of Mo to a steel with a reduced level of C, during tension of the steel sheet in the direction perpendicular to rolling, a low yield point of at most 300 MPa, which is a suitable for application to automotive panels, is realized. Furthermore, they found that by maintaining this steel in a prescribed temperature range after annealing, a suitable amount of austenite is retained. By forming a metal structure substantially of ferrite and a bainite/martensite hard phase and retained austenite, adequate workability can be guaranteed without a deterioration in strain aging properties.
According to one form of the present invention, a steel for use in forming high strength steel sheet comprises, in mass %, C: at most 0.04%, Si: at most 0.4%, Mn: 0.5-3.0%, P: at most 0.15%, S: at most 0.03%, Al: at most 0.50%, N: at most 0.01%, and Mo: 0.01-1.0%.
The steel may further include at least one of Cr: less than 1.5%, Ti: at most 0.15%, Nb: at most 0.15%, and B: at most 0.01%.
In preferred embodiments, the steel has a metal structure containing retained austenite with a volume ratio of at least 0.5% and less than 10%, and a remainder which is a multi-phase structure of ferrite and a hard phase of at least one of bainite and martensite.
The steel may be formed into a high strength cold rolled steel sheet suitable for use as an automotive panel. In preferred embodiments, in a tensile test in a direction perpendicular to the rolling direction of the cold rolled steel sheet, the yield point is at most 300 MPa, the amount of work hardening with a 2% prestrain and the amount of BH are both at least 30 MPa, and the yield ratio is at most 75%.
The cold rolled steel sheet may be subjected to zinc coating by a variety of plating methods to form a zinc-coated steel sheet.
According to another form of the present invention, a method of manufacturing a high strength galvanized steel sheet includes casting a slab of the above-described steel, performing hot rough rolling either directly or after heating to a temperature of at most 1300xc2x0 C., commencing hot finish rolling either directly or after reheating or holding, completing finish rolling at a temperature of at least 780xc2x0 C., performing coiling after cooling to a temperature of 750xc2x0 C. or below at an average cooling rate of at least 3xc2x0 C./second, optionally performing cold rolling either directly or after scale removal, heating to an annealing temperature of at least 700xc2x0 C. and then cooling to a temperature of 600xc2x0 C. or below at an average cooling rate of at least 3xc2x0 C./second, holding in a temperature range of 450-600xc2x0 C. for at least 10 seconds, performing hot dip galvanizing after cooling, and then optionally carrying out alloying.
According to another form of the present invention, a method of manufacturing a high strength steel sheet includes casting a slab of the above-described steel, performing hot rough rolling either directly or after heating to a temperature of at most 1300xc2x0 C., commencing hot finish rolling either directly or after reheating or holding, completing finish rolling at a temperature of at least 780xc2x0 C., performing coiling after cooling to a temperature of 750xc2x0 C. or below at an average cooling rate of at least 3xc2x0 C./second, optionally performing cold rolling either directly or after scale removal, heating to an annealing temperature of at least 700xc2x0 C. and then cooling to a temperature of 600xc2x0 C. or below at an average cooling rate of at least 3xc2x0 C./second, holding in a temperature range of 250-600xc2x0 C. for at least 10 seconds, and then cooling. If desired, the resulting steel sheet may be electroplated with a metal or an alloy having zinc as a primary component to obtain a high strength zinc-coated steel sheet.
A steel according to the present invention can be used to form a cold rolled steel sheet, or a zinc-coated steel sheet formed from either a cold rolled steel sheet or a hot-rolled steel sheet. In the present invention, any type of Zn-based plating can be used. Zinc-coated steel sheet according to the present invention can be produced by various types of manufacturing methods such as hot dip plating, electroplating, vapor deposition plating, and flame spraying. The plating composition can be, for example, pure Zn, a composition having Zn as a primary component such as Znxe2x80x94Fe, Znxe2x80x94Ni, Znxe2x80x94Al, Znxe2x80x94Mn, Znxe2x80x94Cr, Znxe2x80x94Ti, or Znxe2x80x94Mg, or it may be a composition including one or more other alloying elements and impurity elements for improving corrosion resistance or other property, such as Fe, Ni, Co, Al, Pb, Sn, Sb, Cu, Ti, Si, B, P, N, S, or O. In addition, fine ceramic particles such as SiO2 or Al2O3, oxides such as TiO2 or BaCrO4, or an organic polymer such as an acrylic resin may be dispersed in the plating layer. The plating may have a uniform composition in the thickness direction of the plating layer, or the composition may vary continuously or layer by layer. For a multi-layer plated steel sheet, the plating composition of the outermost layer may be pure Zn or one having Zn as a primary component such as Znxe2x80x94Fe, Znxe2x80x94Ni, Znxe2x80x94Al, Znxe2x80x94Mn, Znxe2x80x94Cr, Znxe2x80x94Ti, or Znxe2x80x94Mg, it may further include one or more alloying elements or impurity elements for improving a property such as corrosion resistance, and if necessary fine ceramic particles such as SiO2 or Al2O3, oxides such as TiO2 or BaCrO4, or an organic polymer such as an acrylic resin may be dispersed in the plating layer.
Some examples of a plated steel sheet are a hot-dipped galvanized steel sheet, a vapor deposited zinc-coated steel sheet, hot-dipped iron-zinc galvannealed steel sheet, a hot-dipped zinc-coated steel sheet in which the plating is an alloy of zinc as a primary component with aluminum, iron, or the like, hot-dipped galvannealed steel sheet in which the lower layer in the cross-sectional direction of the plating is alloyed (generally referred to as a half-alloy), a plated steel sheet having on one side a hot-dipped galvannealing which is an alloy of iron and zinc and having on its other side a hot-dipped galvanizing, a steel sheet in which plating of zinc or plating having zinc as a main component and containing iron or nickel is plated atop one of the above-described platings by electroplating, vapor deposition plating, or the like, an electrodeposited zinc-coated steel sheet, an electroplated steel sheet plated with an alloy of zinc, nickel, chromium, or the like, electroplated steel sheet having a single alloy layer or multiple alloy layers, or a steel sheet plated by vapor deposition plating of zinc or a zinc-containing metal. In addition, it may be a plated steel sheet in which ceramic fine particles such as SiO2 or Al2O3, fine oxide particles such as TiO2 or BaCrO4, or organic polymers are dispersed in a zinc or zinc alloy plating.
The reasons for the limitations on the steel composition according to the present invention and on the manufacturing conditions for a steel sheet according to the present invention will be described below in detail. When referring to the steel composition, unless otherwise specified, xe2x80x9c%xe2x80x9d means xe2x80x9cmass %xe2x80x9d.
(A) Steel Composition
C: C is necessary in order to obtain a multi-phase structure and retained austenite. However, if the C content is greater than 0.04%, the yield strength of the steel sheet becomes too high, and it is not suitable for use for automotive panels. Accordingly, the C content is made at most 0.04%. Preferably it is at least 0.001%, more preferably it is at least 0.005%, and still more preferably it is at least 0.01%.
Si: Si is effective for increasing strength, but it brings about a decrease in toughness and a worsening of the surface condition. Furthermore, it stabilizes austenite, so the amount of retained austenite increases. During the manufacture of a zinc-coated steel sheet, Si impedes the wettability of plating and impedes galvannealing treatment (alloying treatment). Accordingly, the upper limit on the Si content is 0.4%. The upper limit is preferably 0.2% and more preferably 0.1%.
Mn: The addition of at least 0.5% of Mn is necessary in order to obtain a multi-phase structure. However, if the Mn content exceeds 3.0%, the yield strength of the steel sheet becomes too high, and it becomes unsuitable for use for automotive panels. Accordingly, the Mn content is 0.5-3.0%. Preferably it is 1.0-2.0%.
P: P is advantageous for increasing strength, but addition of a large amount of P worsens weldability. Accordingly, the upper limit on the P content is 0.15%. The P content is more preferably less than 0.05%. The total amount of P and C, which worsens weldability, is preferably less than 0.08% and more preferably less than 0.05%.
S: S causes hot embrittlement and deteriorates surface quality, so it is an undesirable element. Therefore, the amount thereof is preferably as low as possible, and the S content is made at most 0.03%.
N: N diffuses rapidly, so it has a large affect on a deterioration of properties caused by aging at room temperature. Accordingly, the N content is preferably low, and the upper limit is made 0.01%.
Al: Al is added in order to carry out deoxidation of steel at the time of preparation of a molten steel. However, the effect of Al saturates when a large amount thereof is added, and costs merely increase without a corresponding improvement in properties, so the upper limit on the Al content is made 0.50%. Preferably the Al content is at most 0.10%. Al also has the effect of reducing the amount of solid solution N by forming a nitride, so preferably at least 0.005% of Al is added.
Mo: In the present invention, by adding at least 0.01% of Mo, a multi-phase structure steel sheet including retained austenite having a low yield strength suitable for automotive panels can be obtained. However, if the Mo content exceeds 1.0%, the yield strength of the steel sheet becomes too high, so the upper limit is made 1.0%. Accordingly, the amount of Mo which is added is 0.01-1.0% and preferably 0.1-0.6%.
B: B has the effect of reducing solid solution N by forming a nitride, so it may be added if necessary. However, the effect of B saturates when a large amount thereof is added, and costs merely increase without a corresponding improvement in properties, so the upper limit is made 0.01%.
Cr: Cr promotes formation of a multi-phase structure, so it may be added if necessary. However, the effect thereof saturates when 1.5% or above is added, so the Cr content is made less than 1.5%. Preferably it is less than 1.0%.
Ti: Ti has the effect of fixing N, which promotes aging deterioration, so Ti may be added if necessary. However, when the Ti content exceeds 0.15%, there is the problem that the yield point increases due to precipitation hardening. Accordingly, the Ti content is made at most 0.15%. Preferably it is at most 0.03%.
Elements other than those described above may be added in an amount within a range in which they do not cause a deterioration in the properties which the present invention attempts to improve. For example, Cu, Ni, and the like may be added each in an amount of at most 0.1%, Nb may be added in an amount of at most 0.15%, and V, Ca, Sn, Sb, and the like may also be added,each in an amount of at most 0.03%.
(B) Metal Structure
In a preferred embodiment, the metal structure of a steel according to the present invention contains retained austenite with a volume ratio (below xe2x80x9c%xe2x80x9d with respect to the metal structure refers to the volume ratio) of at least 0.5% and less than 10%. The problem of a low r-value and poor formability of a multi-phase structure steel sheet can be solved by increasing the elongation through the TRIP (transformation induced plasticity) effect of retained austenite. In order to obtain this effect, it is necessary for the amount of retained austenite to be at least 0.5%. A high degree of work hardening is obtained from the TRIP effect, so the amount. of work hardening with a 2% prestrain, which is effective for dent resistance, is also high. However, if the volume ratio is 10% or above, large strains resulting from a large amount of work hardening are excessively obtained, the strength becomes too high, and ductility decreases, so it becomes easy for yield point elongation (YPE), which worsens surface quality, to occur. Preferably the volume ratio of retained austenite is in the range of 0.5-5% and more preferably it is 0.5-4%.
In this preferred embodiment, it is desirable for the remainder of the metal structure to be a multi-phase structure of ferrite and a hard phase. The hard phase preferably has a Vickers hardness of at least 200 HV and it is bainite and/or martensite, but it is preferably primarily martensite.
By forming a multi-phase structure of ferrite and a hard phase, a high strength cold rolled steel sheet or a high strength zinc-coated steel sheet can be obtained which has a yield point of at most 300 MPa, work hardening(WH) with a 2% prestrain and BH each of at least 30 MPa, and a yield ratio of at most 75% during tension in a direction perpendicular to the rolling direction, and which has excellent strain aging properties and excellent formability and shape retention. Preferably the yield point is at most 280 MPa, the tensile strength is at most 510 MPa, the amount of WH is at least 50 MPa, and the amount of BH is at least 50 MPa. More preferably the yield point is at most 250 MPa.
(C) Hot Rolling Conditions
Hot rough rolling is commenced directly after continuous casting or after heating to a temperature of at most 1300xc2x0 C. or after holding at the cast slab temperature. After the completion of hot rough rolling, finishing rolling is commenced either immediately after rough rolling or if necessary after performing reheating of the rough bar or performing the holding. Finish rolling is completed at a temperature of at least 780xc2x0 C., and coiling is performed after cooling to a temperature of 750xc2x0 C., or less at an average rate of at least 3xc2x0 C. per second.
Hot rough rolling of a slab which is manufactured by continuous casting may be directly commenced at a high temperature, or rolling may be commenced after heating to at most 1300xc2x0 C. or after holding. When heating or holding is carried out, the temperature is made at most 1300xc2x0 C. in order to coarsen precipitates and to increase the r-value. It is preferable to decrease the temperature, and it is preferably at most 1200xc2x0 C. and more preferably at most 1100xc2x0 C.
After the completion of rough rolling, finish rolling is commenced, and rolling is completed at a finishing temperature of at least 780xc2x0 C. As described above, if the slab heating temperature is decreased, it is difficult to maintain the finishing temperature. As a means of avoiding this problem, it is extremely effective to reheat or hold the temperature of all or a portion of the rough bar prior to beginning finish rolling. As a method of heating or holding, the rough bar can be wound into the shape of a coil and placed into a furnace, or the rough bar can be heated by a rough bar heater which heats the rough bar by induction heating, it can be heated with a gas burner, or a conductive heating method in which a current is passed directly through the rough bar can be used. A heating method using a rough bar heater is particularly preferred.
Prior to finish rolling, it is advantageous to join a plurality of rough bars together and then to carry out continuous rolling because finishing can be carried out at a high speed in a short period of time without too great a decrease in speed.
If the finishing temperature falls below 780xc2x0 C., the amount of an unsuitable texture increases in the hot rolled steel sheet and the r-value of the final product decreases, which is undesirable. Preferably the finishing temperature is at least 820xc2x0 C. and more preferably at least 850xc2x0 C.
After finish rolling, cooling is carried out to 750xc2x0 C. or below at an average cooling rate of at least 3xc2x0 C. per second, and then coiling is carried out. Rapid cooling at a rate of at least 3xc2x0 C. per second to 750xc2x0 C. or below is carried out in order to refine ferrite crystal grains. If the crystal grains are coarse, carbides easily precipitate after annealing, and retained austenite and a hard phase of bainite or martensite are not obtained. In order to refine the crystal grains or obtain a bainite structure, the cooling rate is preferably 10xc2x0 C. per second or higher, and the coiling temperature is preferably 300-600xc2x0 C. and more preferably 400-550xc2x0 C.
(D) Annealing Conditions
After hot rolling, scale removal is carried out, and if necessary, cold rolling is performed. Scale removal is normally carried out by pickling. Either before or after scale removal, leveling may be carried out by skin pass rolling or with a leveler.
Cold rolling can be carried out by ordinary methods. The reduction is preferably at least 40%, since this provides a suitable texture.
After cold rolling, annealing is carried out by continuous annealing or with a continuous hot dip galvanizing line. Annealing is carried out by heating to at least 700xc2x0 C., and normally by heating to at least 720xc2x0 C. which is above the Ac1 point. In order to adequately guarantee a hard phase for preventing a deterioration in strain aging properties, the annealing temperature is preferably at least 780xc2x0 C. and more preferably at least 820xc2x0 C.
Subsequent to annealing, after cooling is carried out to a temperature of 600xc2x0 C. or below at an average cooling rate of a least 3xc2x0 C. per second, it is important to perform holding in a range of 250-600xc2x0 C. for at least 10 seconds. If the cooling rate is less than 3xc2x0 C. per second, austenite can be decomposed into pearlite or cementite during the cooling process, so a multi-phase structure having satisfactory room temperature aging properties is not obtained. Preferably the cooling rate is 8-120xc2x0 C. per second. After cooling, it is important to perform holding in a range of 250-600xc2x0 C. for at least 10 seconds. Due to this holding, austenite does not break down into cementite, and the austenite is stabilized by concentration of austenite stabilizing elements such as C. Preferably the holding is carried out in a temperature range of 300-600xc2x0 C. for 10-18 seconds, and more preferably in the range of 450-600xc2x0 C. for 10-60 seconds.
When manufacturing a hot-dipped galvanizing steel sheet, if the holding temperature is less than 450xc2x0 C., reheating must be carried out, which is not desirable, so the holding temperature is preferably made 450-600xc2x0 C.
When carrying out holding, the temperature may be maintained at a constant temperature, or the temperature may be decreased at a rate of at most 2xc2x0 C. per second during holding.
After holding, the steel sheet can be cooled at a rate of at least 3xc2x0 C. per second as is or after carrying out hot dip galvanizing or after further carrying out lead-zinc alloying treatment, i.e., galvannealing. If the cooling rate is less than 3xc2x0 C. per second, austenite breaks down into pearlite or cementite during the cooling process, and a multi-phase structure having good strain aging properties is not obtained.
Next, skin pass rolling may be carried out with a reduction of at most 2.0% in order to adjust the surface roughness or to carry out leveling. Steel sheet which has been cooled as is after holding may have its surface electroplated with plating primarily comprising zinc. A lubricating conversion coating may be formed or oil may be applied to the zinc-coated steel sheet. From the standpoint of sliding properties, the roughness of the surface is preferably an average surface roughness Ra of at most 1.2 micrometers and more preferably at most 1.0 micrometers.